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Bayer Butyl, Bayer Bromobutyl, Bayer Chlorobutyl
A9.3 Compounding

Selection of the suitable Bayer Butyl grade
The best choice for a certain application often entails finding a compromise between the desired properties and the rate of cure. The regular grades, as stated, are available with degrees of unsaturation of 0.7 to 2.2 mole %. With increasing unsaturation
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the rate of cure increases
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the ozone resistance of the vulcanizate decreases
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the heat resistance increases.
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The last-mentioned effect is associated (in the range of low unsaturation from 0.7 to 2.2 %) with the denser crosslinking that is possible with materials that have fairly high isoprene contents.

The viscosity of the polymer can be chosen also. As with other rubbers, the Mooney viscosity of the polymer (Tables A9-4 and A9-5) has a strong influence on the processing behavior, low viscosity grades being easier to process. If the compound needs fairly good dimensional stability or shape retention, the grades with higher viscosity are preferred, e.g. Bayer Butyl 301 for tire inner tubes. Such grades have the additional advantage that heavier loading with oils or plasticizers is possible; this facilitates processing, reduces costs and improves the recovery of the vulcanizate at low temperatures. With regard to green strength, precrosslinked butyl rubber (Table A9-3) naturally behaves particularly well.

The aspects which affect the selection of halogenated butyl rubbers are connected primarily with vulcanization (rate of cure, achievable degree of crosslinking, covulcanization, variability with respect to the vulcanization system), as will be explained in the following sections.

The faster and more effective vulcanization of the halogenated butyl grades is a particular advantage, e.g. with respect to the heat resistance of vulcanizates. Furthermore, halogenated butyl rubbers can be crosslinked with relatively lower levels of vulcanizing agents. This is important for applications in the pharmaceutical sector, where articles with low extractability of the various chemicals are required. In addition, bromobutyl rubber shows the best adhesion to reinforcing substrates (in combination with RFL systems, see chapter D10) and also with brass-coated steel cord. Here normal IIR and also CIIR are inferior.

In connection with the choice of a suitable butyl rubber grade, it is worth mentioning that the low permeability to many gases and liquids is characteristic of all grades. This is the most important property of butyl vulcanizates (see section A9.5).

Information concerning compliance with FDA and BgVV regulations for use in contact with foodstuffs can be obtained on request from the Health, Safety, Environment and Quality Department (HSEQ) in the Business Planning and Administration Section of the Rubber Business Group.
 

Blends of butyl rubber with other rubbers
Special attention should be given to the blend technology of butyl rubber. As mentioned, the regular grades do not covulcanize with other highly unsaturated rubbers. The halogenated butyl rubber grades, on the other hand, are covulcanizable with NR, BR, CR and others. In this respect, bromobutyl rubber clearly behaves even more favorably than chlorobutyl rubber. Unlike regular butyl rubber, blends of halogenated butyl rubber with other polymers (e. g. NR) show improved tack and also - this is often why such blends are used - clearly enhanced cohesion (ply adhesion) with other compounds after curing.

Chlorobutyl rubber is considerably more active in crosslinking than regular IIR; bromobutyl rubber, in turn, is more active than the chloro derivative. Therefore, if one replaces BIIR with CIIR in formulations based on BIIR, longer cure times can be expected. In such cases the type and level of the vulcanizing agents may have to be adjusted in order to optimize the vulcanizate properties.

Blending halogenated butyl rubber with, e.g., NR understandably necessitates a compromise with regard to properties. This also applies to the gas permeability of HIIR (halogenated butyl rubber) blends, for example.
 

Vulcanization chemicals
In view of the low level of unsaturation of butyl rubber, acceptable cure times depend on the use of highly active accelerator systems. For example, combinations of Vulkacit Thiuram (TMTD) and Vulkacit Merkapto (MBT) can be used, if necessary with addition of dithiocarbamates. With such sulfur-accelerator systems, a balanced relationship between processing safety, rate of cure and mechanical properties is often possible. Systems based on quinone dioxime (plus oxidizing agents, such as red lead, lead dioxide or MBTS) can be used when fast and effective crosslinking and good heat and ozone resistance are sought, e.g. for medium and high voltage electrical insulation. Nevertheless, quinone dioxime curing has become largely obsolete in Europe.

Maximum heat resistance, an absolute necessity for curing bags and bladders used in tire manufacture, is achieved by the use of resin cures. For this purpose, halogenated or non-halogenated dimethylol phenol resins are used (see section A9.5).

With halogen-containing butyl rubbers, zinc oxide is the appropriate vulcanizing agent. It is not used alone, but - to increase the degree of cure - with sulfur (e.g. 0.5 pbw), and stearic acid (<1 pbw) and also with low levels of Vulkacit Merkapto, Vulkacit DM or sulfenamides. Sulfur donors can be used also, and lead to still better heat resistance and lower compression set. Peroxide vulcanization, with its known advantages and disadvantages, is also suitable for BIIR. With these grades, resin cures do not give the high heat resistance which is achieved with the regular grades. Certain antioxidants act as vulcanizing agents in compounds based on halogen-containing butyl rubbers (see "quantities").

Magnesium oxide and calcium stearate, if used along with the above-mentioned accelerators in halogenated butyl rubber, act as retarders. Retarders must be used with caution as they increase not only the scorch time but also the cure time; they may also reduce the final degree of crosslinking.

Further possibilities for BIIR are: m-phenylene-bis-maleimide as vulcanizing agent plus zinc oxide; hexamethylene diamine carbamate without zinc oxide and without sulfur for pharmaceutical applications. Information about this complex field will be given on request.

Vulcanizing agent quantities (pbw) for IIR
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Normal quantities (e. g. for tire inner tubes)

1.5

Vulkacit Thiuram

0.5

Vulkacit Merkapto

1.7

Sulfur

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For better resistance to aging

2

Vulkacit Thiuram

0.4

Vulkacit Merkapto

1

Sulfur

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System with little or no nitrosamine content

1

Vulkacit NZ

0.2

Vulkacit D

0.5

Vulkacit Merkapto

7

BIIR (Bayer Bromobutyl 2030)

1.5

Sulfur

(possibly plus a small quantity of TMTD)
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Quinone dioxime cure

1

Stearic acid

2

p-quinone dioxime

6

Red lead (without ZnO)

or

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5

Zinc oxide

6

pp'-dibenzoyl-p-quinone dioxime

10

Red lead (without stearic acid)

The first cure system gives an earlier onset of cure and higher rate of cure than the second.
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Resin cure

5

Chlorine donor (Baypren 110 or SnCl2 . 2 H2O)

5

Zinc oxide

7

Dimethylol phenol resin (e.g. SP 1045, Schenectady)

For low compression set, halogenated butyl rubber or SP 1055 resin can be used.
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Vulcanizing agent quantities (pbw) for CIIR
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Typical system for blends with 20 pbw NR

3

Zinc oxide

1.5

Vulkacit DM

0.3

Vulkacit Thiuram

0.5

Sulfur

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System with alkylphenol sulfide

5

Zinc oxide

0.75

Vulkacit DM

1.25

Vultac 5 (Pennwalt)

0.5

Magnesium oxide

0.5

Sulfur

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System with antidegradant as vulcanizing agent

3

Diphenylamine-acetone reaction product 
(e.g. Aminox, Uniroyal) + 2-mercaptobenzimidazole
(e.g. Vulkanox MB) 1 : 1

3

Zinc oxide (without sulfur)

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Resin cure

5

Zinc oxide

0.3

Magnesium oxide

4.5

Dimethylol phenol resin (e.g. SP 1045, Schenectady)

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Vulcanizing agent quantities (pbw) for BIIR
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Typical system for inner liners

3

Zinc oxide

1

Stearic acid

1.3

Vulkacit DM

0.5

Sulfur (MgO acts as retarder)

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System with antidegradant as vulcanizing agent

2

N,N'-di-b-naphthyl-p-phenylene-diamine
(e.g. Age Rite White, Vanderbilt)

3

Zinc oxide (without sulfur)

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Resin cure

5

Zinc oxide

3

Dimethylol phenol resin (e.g. SP 1045, Schenectady)

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Antidegradants
The stabilizers contained in the material from the polymerization stage onwards are as a rule adequate. Therefore IIR formulations generally require no additional antidegradants. As regards ozone protection, see section A9.5.

Fillers
For most compounds used in practice, reinforcing fillers are specified to achieve smooth processing and appropriately high modulus and strength properties in the vulcanizate. Carbon black together with an appropriate quantity of process oil gives the best balance as regards processing, especially by extrusion and calendering, and the physical properties of the vulcanizate. However, light-colored fillers such as clay, talc, silica fillers and whiting are also used when inferior vulcanizate properties can be accepted. Bayer Bromobutyl and Chlorobutyl generally respond to carbon blacks and mineral fillers in the same way as Bayer Butyl and other synthetic rubbers. However, the high reactivity of the allylic bromine in bromobutyl rubber does enable good bonding to mineral fillers such as silica through the use of silanes such as aminopropyl triethoxy silane or TESPD or TESPT. This can result in compounds containing butyl rubber with high filler reinforcement. More information will be provided on request.

Plasticizers
Plasticizers are used in butyl compounds to aid processing, reduce the hardness and modulus of the vulcanizates, improve the elastic properties and set, and also to reduce costs. Quite high plasticizer loadings are needed for good low temperature resilience, such as that desired, for example, in tire inner tubes for use under severe winter conditions. Mostly mineral oil plasticizers are used. Paraffinic and naphthenic products are preferred, as butyl rubber has a low solubility parameter. Ester plasticizers (see chapter D7) are sometimes used if there are unusual requirements with regard to low temperature behavior. Plasticizers and other additives with olefinic unsaturation may disturb the vulcanization and should be avoided.

Tackifiers can improve adhesion and increase flow at high temperatures. They should be selected according to melting point, a property which may influence not only their incorporation but also the final properties of the article. Polyterpenes, terpene-phenolic derivatives and special synthetic hydrocarbons are effective.

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